The present invention relates to an improvement in the process for decorating glass-ceramic materials with patterned stains. More particularly, the invention relates to a mode of staining glass-ceramics whereby greater uniformity of coloration is achieved without causing the color of the stain to diverge from the customary gray or blue-gray.
Glass-ceramics are articles in which a major crystalline phase is dispersed in a glassy matrix, and which are individually produced by forming an article from a crystallizable glass and subsequently subjecting the glass article to heat treatment in order to induce crystallization within the body of the glass. The resulting glass-ceramic products are useful because of their relatively low coefficient of thermal expansion and high resistance to thermal shock, which lend the glass-ceramic articles to use in high temperature applications. A particular application for which the use of glass-ceramics has become popular is as cooking surfaces in domestic appliances such as stove tops and microwave oven shelves. Additional information regarding the basic nature of glass-ceramic compositions and methods of making may be found in U.S. Pat. No. 2,920,971 to S. D. Stookey and in U.S. Pat. No. 3,625,718 to R. W. Petticrew.
It is often desired to apply a decorative pattern to a glass-ceramic article, and in order to provide such a decoration with abrasion resistance, it is preferred to impart the decoration of the glass-ceramic by staining, whereby the coloration is contained within the surface portion of the articles. Staining is carried out by applying a stain composition to the crystallizable glass surface in the desired pattern, such as by silk-screening, and then maintaining the crystallizable glass at an elevated temperature to migrate stain-producing ions into the surface portion of the glass-ceramic. For domestic appliance applications, the preferred staining compositions include palladium as the staining agent, which usually produces a neutral gray or blue-gray colored stain without detrimentally affecting the luster of the glass-ceramic surface. Suitable palladium stains are commercially available in the form of palladium resinates in an organic solvent to which significant amounts of inert extender, such as titanium dioxide, may be added. Prior to this invention, crystallization and staining usually have been carried out simultaneously in one heat treatment step.
Occasionally, it is found that glass-ceramic articles stain-decorated in the customary manner with palladium are stained non-uniformly. That is, some areas of the stained pattern achieve a darker gray coloration than other areas of the stained pattern. These non-uniformly stained glass-ceramic articles were unsightly and had to be discarded. It has been theorized that the non-uniformity of the stained patterns was caused by inhomogeneities in the composition of the glass-ceramic articles arising from the melting and forming process. Glass-ceramic compositions require relatively high melting temperatures and are, therefore, prone to losing volatile constituents of the composition during melting, and their melting may be accompanied by leaching of elements from the refractory furnace walls into the melt, both of which lead to inhomogeneities in the product withdrawn from the furnace. It has been found that the darker gray areas in the non-uniformly stained patterns correspond to areas of the glass in which the zirconia concentration is slightly greater than that of the bulk of the glass-ceramic articles.
Various methods and compositions for stain-decorating glass-ceramics are disclosed in the following U.S. patents:
U.S. Pat. No. 3,313,644--J. W. Morrissey PA1 U.S. Pat. No. 3,528,847--P. Grego et al. PA1 U.S. Pat. No. 3,775,154--P. Grego et al. PA1 U.S. Pat. No. 3,816,161--M. J. Buckley PA1 U.S. Pat. No. 3,852,052--H. M. Demarest, Jr. PA1 U.S. Pat. No. 3,940,531--H. M. Demarest, Jr.